This is done by means of a permanent or pulsed low-voltage current flow by way of the connecting lines between the safety combination and the emergency stop facility. Since it can happen that an emergency stop facility embodied as an emergency stop button is mechanically blocked and as a result cannot be activated, it is normal practice to use what are known as dual-channel safety combinations, to which two emergency stop buttons, which are positioned in direct proximity to each other and are activated by way of a common contact button, are connected in parallel. Generally with dual-channel safety combinations, the connecting line pairs, made up of the lines to and from the two emergency stop buttons and also referred to as channels, run parallel to each other. This means there is a risk of what is known as a cross connection, wherein the current-carrying connecting lines are connected to each other. This means that the safety combination could no longer determine whether or not an emergency stop button had been activated.
As a counter-measure what is known as cross connection recognition is used to recognize a current flow between the two channels. This is done by pulsing the current flow through the two emergency stop buttons arranged parallel to each other one after the other in an alternating manner. If a current flow is determined at an input of the safety combination connected to a first emergency stop button at a time when a current pulse is emitted to an output of the safety combination connected to the second emergency stop button, this indicates a cross connection.
One difference when using electronic sensors in place of emergency stop buttons is that the function of the sensor or its outputs is checked regularly, by self-monitoring of the electronic sensor. To this end the electronic sensor transmits a low-voltage signal, which it switches off briefly at regular intervals. At the same time it is checked internally within the sensor whether or not the low voltage has been switched off at the output. If the low voltage has been switched off, the signal is supplied to the output again. The brief switching off, checking and switching on again take place so quickly that the operation cannot be recognized by the safety combination.
For economical reasons it is not expedient when producing safety combinations to differentiate between single-channel and dual-channel safety combinations for electronic sensors initiating an emergency stop and emergency stop buttons. Also it frequently happens that already installed safety combinations are changed from emergency stop buttons to electronic sensors or vice versa, since requirements change over time.
For this reason safety combinations are mainly embodied as dual-channel. These dual-channel safety combinations have an internal switchover between single-channel and dual-channel operation. During single-channel operation an electronic sensor or an emergency stop button can be connected. During dual-channel operation the safety combination carries out cross connection recognition by means of pulsed current flows. Also bidirectional bridges are arranged in the safety combination, with which the two channels have to be connected electrically to each other in single-channel operation.
If a dual-channel electronic sensor is to be connected to a dual-channel safety combination thus equipped, cross connection recognition must be carried out as with two emergency stop buttons connected in parallel. While in the case of emergency stop buttons the safety combination monitors for cross connection up to the emergency stop buttons, electronic sensors connected to the safety combinations provide cross connection monitoring up to the terminals of the safety combination itself, since dual-channel sensors have to monitor themselves in any case, just like single-channel sensors. To facilitate this, the safety combination must be switched to single-channel operation, with the result that the cross connection recognition of the safety combination is deactivated and the electrical connection between the two channels is broken by removing or switching over at least one bridge.
This procedure is prone to error, time-consuming and therefore expensive and represents a potential source of error and therefore a safety risk.